When a nuclear power plant accident like the one mentioned above occurs, a large quantity of contaminated soil (intermediate and low level radioactive waste) may be found on houses, schools, agricultural land, and forest roads in an area 20 to 30 km around the nuclear power plant.
Since the Fukushima nuclear power plant accident, there has been a stronger need than ever before for a method for disposal of radioactive waste which enables a large quantity of intermediate and low level radioactive waste to be safely transported and stored and which enables final disposal of waste after the radiation level is reduced to a natural radiation level or national predetermined reference value (e.g., ICRP recommendation: a yearly radiation dose of 20 mSv).
The fission product 137Cs (55) contained in radioactive waste is turned by the β decay into 137Ba (56) which emits gamma rays. In this regard, Cs has a half-life of 30.17 years, thus requiring a long-term storage. Furthermore, the gamma rays emitted from 137Ba (56) themselves have an exceptionally high transmittance when compared with other α rays or β rays. Typically, as a shield for gamma rays, heavy concrete (having a specific gravity of 3.5 to 4.3; up to 2.3 for ordinary concrete) which has aggregates of iron ore and iron scrap in concrete have been used (see Encyclopaedia Chimica Editing Committee (ed.) (Jul. 31, 1962) “Encyclopaedia Chimica 8”, p.617, Kyoritsu Shuppan). However, the storage container or capsule that employs the heavy concrete as a shield increases in weight, and thus had problems in transportation and storage. Furthermore, the concrete is fundamentally porous, thus completely preventing leakage of radioactive nuclides is difficult. Thus, radioactive nuclides had to be covered with a closely packed shield or buried deep in the ground, making it difficult to ensure storage sites or the like.
In this context, the development of a method for disposal of radioactive waste has been desired, the method enabling the intermediate and low level radioactive waste to be reduced in radiation level until the waste can be reused as landfill or the like and preventing the leakage of radiation nuclides in the ground.
However, the inventors of the prevent invention have not heard of known techniques for such a method for disposal of radioactive waste.
Although having no effects on the patentability of the present invention, the following prior art documents exist.
Disclosed in Patent Documents 1 and 2 is “a radioactive waste storage container (radiation shielding structure) composed of constituents including calcium silicate and magnesium oxide, or oxidation calcium and acid phosphate.”
Disclosed in Patent Document 3 is “a radioactive waste disposal container characterized by including: a body material wall constituting skeleton of the container; a corrosion-resistant barrier material wall for covering the outer surface of the body material wall so as to prevent the same from an external corrosive environment; and an insulating material wall which is assembled between the body material wall and the corrosion-resistant barrier material wall and which electrically insulates between the body material wall and the corrosion-resistant barrier material wall.